1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus. More particularly, the present invention relates to an image forming apparatus capable of controlling inside humidity for thereby allowing each constituent device to stably operate in a particular, adequate humidity environment.
2. Description of the Background Art
Generally, an electrophotographic image forming apparatus includes a photoconductive drum or similar image carrier. A charging device, an image transferring device, a developing device and a cleaning device are arranged around the drum. Further, a fixing device, a sheet feeding device, a sheet discharging device and so forth are arranged in the apparatus. These devices are stably operable when humidity inside the apparatus lies in an adequate range, so that due consideration should be given to humidity in the apparatus. Usually, humidity around the drum should preferably be as low as possible while the other devices each have a particular, adequate humidity condition. A relation between each device and humidity will be described hereinafter.
The charging device, configured to uniformly charge the drum to positive or negative polarity, is implemented as, e.g., a charger or a charge roller. The problem with this kind of charging device is that ozone, nitrogen oxides and other gases are produced by discharge. If such discharge products stay around the charging device, then the discharge of the charging device becomes unstable with the result that charging and discharging are made irregular, lowering the quality of a toner image formed on the drum. Further, the discharge products deposited on the drum absorb moisture present in air to thereby lower the surface resistance of the drum, causing a latent image formed on the drum to flow and therefore blur. This is particularly conspicuous when relative humidity is as high as 80% RH or above.
The discharge products mentioned above have high resistance in a low humidity environment and deposit on, e.g., discharging means to thereby make discharge unstable and therefore make charge irregular. This is particularly conspicuous when relative humidity is as high as 30% RH or below.
As stated above, image quality is lowered when humidity around the charging device does not lie in an adequate range. This adequate range is one in which humidity is lower than a value that does not blur an image, but higher than a value that does not bring about irregular charging ascribable to unstable discharge.
As for the image transferring device, when surrounding humidity decreases below an adequate range, defective image transfer occurs due to abnormal discharge. When humidity increases above the adequate range, transferability of a toner image to a sheet or recording medium decreases, causing the toner to be easily scattered around or causing it to easily remain on the drum after image carrier. Thus, image quality is also lowered when humidity around the image transferring device does not lie in the adequate range. This adequate range is one in which humidity is higher than a value that brings about abnormal discharge, but lower than a value that degrades transferability.
In the developing device, when surrounding humidity is low, charge to deposit on the toner excessively increases and obstructs the deposition of the toner on the drum. When humidity is high, the above charge excessively decreases and causes the toner to deposit on the drum in an excessive amount in the event of development. In this manner, image quality is also lowered when humidity around the developing device does not lie in an adequate range, which allows the toner to deposit on the drum in an adequate amount.
When sheets stored in the sheet feeding device is subject to low humidity, it is likely that two or more sheets are fed together due to static electricity. When humidity is high, it is likely that no sheets are fed at all due to a decrease in the hardness of the sheets. Humidity should therefore be maintained in an adequate range in the sheet feeding device also. This adequate range is one that prevents two or more sheets from being fed together due to static electricity and obviates misfeed ascribable to the influence of humidity.
Further, the sheet, carrying the toner image thereon, is curled when driven out to the sheet discharging device due to heat and pressure applied to the sheet by the fixing device. To uncurl such a sheet, it is preferable to humidify the sheet stacked on the sheet discharging device.
In light of the above, Japanese Patent Laid-Open Publication Nos. 5-72871, 8-16073 and 9-81018, for example, propose various schemes for confining humidity inside an image forming apparatus in an adequate range. These conventional schemes, however, need a humidity control mechanism including a heater, a cooler, humidifying means and so forth that consume much power and need an exclusive space for the above mechanism, increasing the overall size of the apparatus. It is therefore impossible to apply the above schemes to a printer, facsimile apparatus or similar image forming apparatus that should be small size. Further, when the humidifying means that uses water is used, daily maintenance, including replenishment of water, sterilization of a water tank, removal of fur and so forth are required, increasing running cost and lowering reliability.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-83129, 8-302218, 9-114321, 10-232591, 10-254330, 11-59933, 11-112709, 2002-72593 and 2002-91105 as well as in Japanese Utility Model No. 2,541,556.